1. Field of the Invention
This invention relates to compressed air operated hydraulic pump apparatus particularly adapted for use with tilting cab control systems to pump hydraulic fluid that moves a tilting cab of a vehicle between lower use and upper access positions.
2. Description of the Prior Art
Vehicle tilting cab control apparatus conventionally includes a hydraulic pump that feeds hydraulic fluid to an extendable cylinder whose extension raises a passenger cab in a tilting fashion from a lower use position to an upper access position where access to the associated vehicle engine for repair or maintenance is permitted. With one type of such a cab vehicle, the extendable cylinder is "single acting" and is supplied pumped hydraulic fluid only to extend its length. Release of the hydraulic fluid with this type of cab permits the cylinder to retract under the force of gravity acting on the cab in a downward direction to move it to its lower use position. Another type of tilting cab vehicle incorporates a "double acting" cylinder that is supplied pumped hydraulic fluid to extend and retract the cylinder in a manner that drives the cab in both directions between its access and use positions.
Manual and hydraulic pumps have previously been utilized to supply pumped hydraulic fluid from a reservoir to the cylinder which moves the cab. Such air operated pumps conventionally include a piston slidably received within a cylinder and alternately moved in opposite directions by a spring and a continuous supply of compressed air. The compressed air is continuously supplied to a first chamber to move the piston against the bias of the spring which is normally positioned in a second chamber on the opposite side of the piston. After the piston has moved a certain amount against the spring bias, the first chamber is exhausted to the environment while the compressed air is continually fed to it. However, the area permitting exhaustion is greater than the area through which the compressed air is supplied so that the net effect is that the spring can move the piston in the opposite direction for a predetermined extent until the exhaustion area is closed. Subsequently, the piston moves in the opposite direction as the pressure again builds up in the first chamber so that the piston is reciprocated. Compressed air thus continually flows to the cylinder in which the piston reciprocates to provide reciprocal motion of a pump plunger that pumps the hydraulic fluid.